1. Field of the Invention
The present invention relates to a plastic foam article comprising a thermoplastic synthetic foam containing at least 10% by volume of open cells which have a polyhedron-like shape and adjoin one another, wherein webs of the polyhedron-like cells arranged in a three-dimensional matrix are intact in terms of shape and at least two walls of each open cell have openings, with maintenance of the mechanical strength of the three-dimensional matrix. The present invention also relates to a process for producing a plastic foam article and use of the shaped article.
2. Description of Related Art
EP-A 0 642 907 discloses shaped articles which are constructed from an open-celled plastic foam sheet which, in contrast to a plastic foam sheet having closed cells, is able to take up liquids in its interior.
Plastic foam sheets have been known for a long time, polystyrene foam sheets in particular have found a wide range of applications. They are used, for example, as packaging and in the building sector especially as insulation material, for example, as wall-paper underlays.
Plastic foam sheets are generally produced by extruding a thermoplastic polymer together with a blowing agent, in particular a blowing gas, from an annular or wide slot die or a coextrusion die. Shortly after leaving the extrusion die, the blowing agent finely distributed in the plastic expands so that very fine bubbles are formed in the plastic foam body. The size of the bubbles obtained depends on the temperature conditions and the extrusion rate. Accordingly, the plastic foam sheet thus produced contains densely packed, closed, gas-filled cells, with the gas enclosed in the cells generally being air and/or residues of the blowing agent. The excellent insulation properties of the plastic foam sheets are essentially based on the densely packed, closed, gas-filled cells which to a certain degree form an "immobile" gas layer, in particular air layer. Such a fixed air layer has a low thermal conductivity.
An entirely different behavior is shown by a plastic foam sheet in which the cells are not closed, but open, i.e., the cells are in fluid communication. Such a plastic foam sheet can, for example, store liquids in a similar manner to a sponge. In such open-celled foams, the cells are connected to one another via the gas phase. In the extreme case, they consist only of cell webs.
The document WO 90/14159 describes an oil absorbent material comprising comminuted polyethylene foam, but this document gives no indication as to how an intact absorbent plastic foam sheet is to be obtained. Application of mechanical pressure to the plastic foam sheet enables a large part of the previously closed cells of the polyethylene foam to be opened. However, this significantly reduces the mechanical strength and the stability of the plastic foam sheet, since the cell walls are partly deformed by the mechanical pressure. Such a sheet can have a proportion of open cells of more than 50%.
EP-A-0 090 507 discloses a dish-shaped packaging for fast food which comprises an open-celled, foamed polymer layer and a closed-cell, polymeric outer layer as liquid barrier layer. The polymer used for both layers is, for example, polystyrene. The formation of the open-celled plastic foam sheet is here carried out in the same manner as the extrusion of the closed-cell outer layer, with the one exception that an excess of nucleating agent, for example, sodium bicarbonate and citric acid, is added in a weight ratio of from 0.8 to 1.2 and from 1.2 to 0.8, but the use of a blowing agent is omitted. Since no blowing agent is used, only a single mixing chamber and a single extrusion screw are required. In this known process, an open-celled polystyrene foam layer having a mean proportion of open cells of from 50 to 90% by volume is obtained. However, this known process does not give a cell structure in which the individual cells are connected to one another; instead, only the cells in the surface of the plastic foam sheet are connected to the adjacent cells lying underneath, but without giving a cell structure in which the cells directly adjoin one another and are separated only by the cell walls.
Japanese Patent 3-57348 discloses a moisture-absorbing packaging material comprising a shaped body which consists of a homogeneous mixture of from 50 to 85% by weight of ethylene polymer and from 15 to 50% by weight of an adsorbent which does not swell on taking up water.
Japanese Patent 54-31026 describes a process for producing a polypropylene foam having an open-celled structure, in which process homopolymers or copolymers of propylene are gelled under pressure and heat, with addition of chlorofluorocarbons as blowing or foaming agents and a conventional nucleating agent. The extruded polypropylene foam has a uniform and fine cell structure with a mean cell diameter of 0.5 mm and a density of about 0.028 g/cm.sup.3. The absorption capacity of this polypropylene foam for water is 3 to 7.6 times the foam density.